1. Field of the Invention
The present invention relates to a SiGe heterojunction bipolar transistor (HBT) and, more particularly, to a SiGe HBT with an improved breakdown voltage-cutoff frequency product.
2. Description of the Related Art
A bipolar transistor is a well-known structure that has an emitter, a base connected to the emitter, and a collector connected to the base. The emitter has a first conductivity type, the base has a second conductivity type, and the collector has the first conductivity type. For example, an npn bipolar transistor has an n-type emitter, a p-type base, and an n-type collector, while a pnp bipolar transistor has a p-type emitter, an n-type base, and a p-type collector.
When the emitter and base are formed from different semiconductor materials, such as silicon and germanium, respectively, the interface is known as a heterojunction. The heterojunction limits the number of holes that can be injected into the emitter from the base. Limiting the number of injected holes allows the dopant concentration of the base to be increased which, in turn, reduces the base resistance and increases the maximum frequency of the transistor.
FIG. 1 shows a cross-sectional view that illustrates an example of a prior-art SiGe heterojunction bipolar structure 100. As shown in FIG. 1, bipolar structure 100 includes a silicon-on-oxide (SOI) wafer 110, which has a silicon handle wafer 112, a buried insulation layer 114 that touches silicon handle wafer 112, and a single-crystal silicon substrate 116 that touches buried insulation layer 114. Silicon substrate 116, in turn, has a heavily-doped, p conductivity type (p+) buried region 120 and a heavily-doped, n conductivity type (n+) buried region 122.
As further shown in FIG. 1, bipolar structure 100 includes a single-crystal silicon epitaxial structure 130 that touches the top surface of silicon substrate 116. Epitaxial structure 130 has a very low dopant concentration, except for regions of out diffusion. For example, a number of p-type atoms out diffuse from p+ buried layer 120 into epitaxial structure 130, and a number of n-type atoms out diffuse from n+ buried layer 122 into epitaxial structure 130. In the present example, epitaxial structure 130 is a very lightly doped, n conductivity type (n−−−) region, excluding the regions of out diffusion.
Bipolar structure 100 also includes a number of shallow trench isolation structures 132 that touch epitaxial structure 130, and a deep trench isolation structure 134 that touches and extends through epitaxial structure 130 as well as silicon substrate 116 to touch buried insulation layer 114. Buried insulation layer 114 and deep trench isolation structure 134 form an electrically-isolated, single-crystal silicon region 136 and a laterally-adjacent, electrically-isolated, single-crystal silicon region 138.
In addition, bipolar structure 100 includes a lightly-doped, p conductivity type (p−) region 140 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to touch p+ buried region 120, and a lightly-doped, n conductivity type (n−) region 142 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to touch n+ buried region 122.
Bipolar structure 100 also includes a p conductivity type sinker region 144 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to p+ buried region 120, and an n conductivity type sinker region 146 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to n+ buried region 122.
Sinker region 144 includes a heavily-doped, p conductivity type (p+) surface region and a moderately-doped, p conductivity type (p) lower region, while sinker region 146 includes a heavily-doped, n conductivity type (n+) surface region and a moderately-doped, n conductivity type (n) lower region.
Further, bipolar structure 100 includes a SiGe epitaxial structure 150 that touches and lies over silicon epitaxial structure 130, a shallow trench isolation structure 132, and p− region 140, and a SiGe epitaxial structure 152 that touches and lies over silicon epitaxial structure 130, a shallow trench isolation structure 132, and n− region 142.
SiGe epitaxial structure 150 has a number of layers including an upper-most layer and a layer that touches and lies below the upper-most layer. The upper-most layer has a center region 154, and an outer region that touches center region 154. Center region 154 has a heavy dopant concentration and a p conductivity type (p+), which results from out diffusion. The outer region, which horizontally surrounds center region 154, has a very low dopant concentration and, in the present example, an n conductivity type (n−−−).
The layer that touches and lies below the upper-most layer, in turn, includes germanium. The layer also has a heavy dopant concentration and an n conductivity type (n+). In addition, SiGe epitaxial structure 150 includes a single-crystal active region, a polysilicon contact region, and a link region that connects the single-crystal active region to the polysilicon contact region.
Similarly, SiGe epitaxial structure 152 has a number of layers including an upper-most layer and a layer that touches and lies below the upper-most layer. The upper-most layer has a center region 156, and an outer region that touches center region 156. Center region 156 has a heavy dopant concentration and an n conductivity type (n+), which results from out diffusion. The outer region, which horizontally surrounds center region 156, has a very low dopant concentration and, in the present example, an n conductivity type (n−−−).
The layer that touches and lies below the upper-most layer includes germanium. The layer also has a heavy dopant concentration and a p conductivity type (p+). In addition, SiGe epitaxial structure 152 includes a single-crystal active region, a polysilicon contact region, and a link region that connects the single-crystal active region and the polysilicon contact region.
Bipolar structure 100 additionally includes an isolation structure 160 that touches SiGe epitaxial structure 150, and an isolation structure 162 that touches SiGe epitaxial structure 152. Isolation structure 160 has an emitter opening 164 that exposes the single-crystal active region of SiGe epitaxial structure 150, and a contact opening 166 that exposes the polysilicon contact region of SiGe epitaxial structure 150. Similarly, isolation structure 162 has an emitter opening 170 that exposes the single-crystal active region of SiGe epitaxial structure 152, and a contact opening 172 that exposes the polysilicon contact region of SiGe epitaxial structure 152.
Bipolar structure 100 further includes a heavily-doped, p conductivity type (p+) polysilicon structure 180 that touches isolation structure 160 and extends through emitter opening 164 to touch the p+ region 154 of SiGe epitaxial structure 150. Bipolar structure 100 also includes a heavily-doped, n conductivity type (n+) polysilicon structure 182 that touches isolation structure 162 and extends through emitter opening 170 to touch the n+ region 156 of SiGe epitaxial structure 152.
P+ polysilicon structure 180 and p+ region 154 form the emitter, the remaining portion of SiGe epitaxial structure 150 forms the n-type base, and the combination of p+ buried region 120, p− region 140, and p-type sinker region 144 form the collector of a pnp SiGe heterojunction bipolar transistor (HBT) 190. In addition, n+ polysilicon structure 182 and n+ region 156 form the emitter, the remaining portion of SiGe epitaxial structure 152 forms the p-type base, and the combination of n+ buried region 122, n− region 142, and n-type sinker region 146 form the collector of an npn SiGe HBT 192.
The maximum (or cutoff) frequency of pnp SiGe HBT 190 is defined in part by the dopant concentration of p− region 140. As the dopant concentration of p− region 140 increases, the collector resistance decreases and the cutoff frequency of HBT 190 increases. On the other hand, as the dopant concentration of p− region 140 decreases, the collector resistance increases and the cutoff frequency of HBT 190 decreases.
The product of the breakdown voltage and the cutoff frequency produces a relatively constant value, which is commonly known as the Johnson limit. Thus, as a result of the Johnson limit, as the dopant concentration of p− region 140 increases, the cutoff frequency of HBT 190 increases while the breakdown voltage of HBT 190 decreases. On the other hand, as the dopant concentration of p− region 140 decreases, the cutoff frequency of HBT 190 decreases while the breakdown voltage of transistor 190 increases.
Similarly, the cutoff frequency of npn SiGe HBT 192 is defined in part by the dopant concentration of n− region 142. Thus, as a result of the Johnson limit, as the dopant concentration of n− region 142 increases, the cutoff frequency of HBT 192 increases while the breakdown voltage of HBT 192 decreases. On the other hand, as the dopant concentration of n− region 142 decreases, the cutoff frequency of HBT 192 decreases while the breakdown voltage of HBT 192 increases.
Advanced low-voltage SiGe HBTs have broken the Johnson limit. These low-voltage SiGe HBTs, however, do not scale well and cannot be used with voltages that are substantially greater than five volts. Thus, there is a need for a SiGe HBT which can break the Johnson limit and handle high voltages.